Alkali metal chlorate, and particularly sodium chlorate, is an important chemical in the cellulose industry, where it is used as a raw material in the production of chlorine dioxide for bleaching of cellulose fibres. Alkali metal chlorate is produced by electrolysis of an electrolyte containing alkali metal chloride according to the overall formula: EQU MeCl+3H.sub.2 O.fwdarw.MeClO.sub.3 +3H.sub.2 (Me=alkali metal)
The process is cyclic, where in a first step the chloride electrolyte is brought to an electrolyser for the formation of hypochlorite, whereupon the solution is brought further to reaction vessels for further reaction to chlorate. Subsequently, chlorate formed is separated by crystallization.
The content of impurities must be low in an electrolyte containing chlorate and chloride ions, if the electrolysis carried out in a chlorate cell shall be energy efficient. Thus, calcium, magnesium and fluoride ions and silicon compounds cause depositions on the cathodes and thereby a higher operating voltage and energy cost. Sulphates and silicon compounds are also known to damage the anodes, thereby increasing the operating voltage and/or decreasing the lifetime.
Calcium ions present in technical salt or raw water from which a chloride brine is prepared, are commonly removed by alkalization at which calcium hydroxide precipitates. Other techniques for purifying chloride brines from calcium ions include precipitation with e.g. carbonate an/or ion exchange with cationic resins. Calcium ions are, however, normally also introduced later in the process. e.g. as impurities in the process water and auxiliary chemicals used. Calcium ions can also be added deliberately, e.g. for removing sulphate ions by precipitating calcium sulphate. Therefore, it is normally necessary to remove calcium ions also from the chlorate electrolyte. Commonly, a portion of the electrolyte is recycled to the brine purifying system.
U.S. Pat. No. 4,946,565 discloses a process for removal of impurities of silicon by precipitation with iron ions. The iron ions can be added either to a chlorate electrolyte or to a brine of alkali metal chloride from which the chlorate electrolyte is produced. When iron silicates are removed separately the resulting precipitate is difficult to filter, thereby necessitating separation by sedimentation. Accordingly, the reduction in silicon content will be limited.
Thus, various methods have been proposed to keep the concentration of calcium ions and silicon compounds in the chlorate process at an acceptable level. Common to the precipitating techniques is a high investment cost for the very fine filters required, low flexibility due to combined purification of brine and electrolyte and/or poor efficiency due to separate removal of the precipitates. The object of the present invention is to provide an efficient and flexible process to remove both calcium ions and silicon compounds from chlorate electrolytes, at a reasonably low investment cost.